Semiconductor laser device and method of manufacturing the same

ABSTRACT

A method of manufacturing semiconductor laser device capable of reducing κL, with manufacturing restrictions satisfied, is provided. In a distributed-feedback or distributed-reflective semiconductor laser device, immediately before burying regrowth of a diffraction grating, halogen-based gas is introduced to a reactor, and etching is performed on the diffraction grating so that each side wall has at least two or more crystal faces and a ratio of length of an upper side in a waveguide direction to a bottom side parallel to a (100) surface is 0 to 0.3. And, a reactive product formed on side surfaces of the diffraction grating and in trench portions between stripes of the diffraction grating at an increase of temperature for regrowth is removed. Therefore, the diffraction grating with reduced height and a sine wave shape is obtained, thereby κL of the device is reduced. Thus, an oscillation threshold and optical output efficiency can be improved.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo. JP 2006-122777 filed on Apr. 27, 2006, the content of which ishereby incorporated by reference into this application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to semiconductor laser devices andmanufacturing techniques thereof, and particularly relates to techniqueseffective when applied to distributed-feedback or distributed-reflectivesemiconductor laser devices for optical transmission apparatuses orinformation storage apparatuses, and a method of manufacturing thereof.

BACKGROUND OF THE INVENTION

For example, as a light source for optical transmission apparatus orinformation storage apparatus, a distributed feedback (DFB) laser usinga refractive-index-modulation diffraction grating, which has narrowspectrum and allows single mode oscillation, is mainly adopted. In a DFBlaser, light output and modulation characteristic significantly changeaccording to κL, product of a coupling coefficient κ of light diffractedin a waveguide direction and an oscillator length L. Therefore, indesigning and manufacturing laser devices, it is important to set κL ata desired value. Here, the coupling coefficient κ is determined byheight of the diffraction grating, distance from an active layer, anddifference in refractive index between a diffraction grating layer and aburied layer (clad layer). In particular, the coupling coefficient κlargely depends on the height of the diffraction grating.

A conventional process of forming a diffraction grating is describedbelow. On an n-InP substrate, an n-InP first clad layer, an n-InGaAlAsfirst optical guide layer, an InGaAlAs active layer, a p-InGaAlAs secondoptical guide layer, a p-InP spacer layer, a p-InGaAsP diffractiongrating layer, and a p-InP cap layer are formed through crystal growth,such as metal organic chemical vapor deposition (MOCVD). In order toincrease a carrier confinement effect, the InGaAlAs active layerincludes a multiple quantum well (MQW) having an InGaAlAs barrier layerand InGaAlAs well layer laminated in a periodic structure.

Furthermore, on the p-InP cap layer, an insulating film, such as asilicon dioxide (SiO₂) film or a silicon nitride (SiN) film, is formed.Then, through photolithography and interference exposure or electronbeam (EB) exposure, a striped pattern is formed in a directionperpendicular to a waveguide. The insulating film is removed through dryetching with fluorinated gas or wet etching with hydrofluoric acidsolution, using the resist pattern as a mask. Then, the resist patternis removed with solvent. Using the insulating film as a mask, the p-InPcap layer and the p-InGaAsP diffraction grating layer are removedthrough dry etching or wet etching to form a rectangular diffractiongrating. Next, a p-InP second clad layer regrowth is performed throughMOCVD or the like.

SUMMARY OF THE INVENTION

Meanwhile, in recent years, achievement of lasers with high outputsstaring at its uncooled operation has been desired not only insemiconductor laser devices for information systems but also in thosefor optical transmission. Lengthening an oscillator and reducing κ inassociation with the lengthening are took up as technical problems.

As has been described above, κ depends on the height of the diffractiongrating. Therefore, in order to reduce κ, the height of the diffractiongrating has to be lowered. However, if the height of the diffractiongrating is too low, large variations in yield of device characteristics,caused by deterioration in etching controllability over the diffractiongrating layer or loss due to thermal decomposition at increase intemperature in burying regrowth or the like, may occur. In conventionalprocess, to avoid these problems, the height of the diffraction gratingafter etching has to be 15 nm or higher. To reduce the κ with themanufacturing restrictions in the height of diffraction gratingsatisfied, the height of the diffraction grating after etching must be20 nm to 30 nm which includes sufficient margin in processcontrollability, and must be lowered immediately before buryingregrowth. In one means for this purpose, the height can be lowered byactively using thermal decomposition at increase of temperature. In thiscase, however, mass transport of the thermally-decomposed layer totrench portions of the diffraction grating produces reaction product.The product is low in crystallinity and may cause deterioration of laserdevice characteristics.

A theoretical value of κL is varied depending on whether the diffractiongrating has a rectangular shape or a sine wave shape, and value of κ ina diffraction grating having a sine wave shape can be reduced. FIG. 5depicts simulation results of a relation between the height of thediffraction grating and κL depending on a difference in shape of thediffraction grating. Note that κL is relative value. It is assumedherein that a composition wavelength (λ) of the InGaAsP diffractiongrating layer is 1.15 μm, and the oscillator length (L) is 500 μm. Aprimary component in a result of Fourier transform of a cross-sectionshape of the diffraction grating in the waveguide direction consideredas a periodic waveform affects the magnitude of κL. Therefore, such aphenomenon as depicted in the drawing occurs. According to this result,by forming the diffraction grating in a sine wave shape, κL can bereduced by 21.5% (π/4) compared with the case of a rectangular shape.Usually, a diffraction grating is formed through dry etching or wetetching. In this case, a shape of the diffraction grating becomerectangular, and it is difficult to form a sine wave shape throughetching. This sine wave shape can be formed through thermaldecomposition at increase of temperature in burying regrowth. In thiscase, however, the above-mentioned problem occurs due to mass transport.

As has been described above, in the conventional semiconductor laserdevice manufacturing technology, the lower limit of height of thediffraction grating is determined by the restrictions in the diffractiongrating forming process, thereby it is difficult to reduce the value ofκ. Also, a reactive product is formed on side surfaces and the trenchportions of the diffraction grating in a process of an increase oftemperature in burying regrowth of the diffraction grating, and causesdeterioration of oscillation threshold and optical output efficiency ofthe device.

An object of the present invention is to overcome the above-describedproblems, and to provide a semiconductor laser device manufacturingtechnology capable of reducing κL with manufacturing restrictionssatisfied.

The above and other objects as well as novel features of the presentinvention will be readily apparent from the description of thespecification and accompanying drawings.

The outline of a representative one of the inventions to be disclosed inthe present application is briefly explained as below.

The present invention is applied to a distributed-feedback ordistributed-reflective semiconductor laser device having diffractiongratings formed in stripes perpendicular to a waveguide direction, andcharacterized by that each diffraction grating has side walls eachhaving at least two or more crystal faces, and a ratio of length of anupper side to a bottom side of the diffraction grating, in a waveguidedirection parallel to a (100) surface, is 0 to 0.3.

Furthermore, the diffraction grating is formed of III-V family compoundsemiconductor layer including at least one of In, Ga, As, and Pelements.

Still further, immediately before burying regrowth of the diffractiongrating, halogen-based gas is introduced to a reactor, and etchingprocess is performed to the diffraction grating to have theabove-described shape. And a reactive product, formed on side surfacesof the diffraction grating and in trench portions between stripes at anincrease of temperature in regrowth, is removed.

The effects achieved by a representative one of the inventions to bedisclosed in the present application is briefly explained as below.

According to the present invention, in the semiconductor laser devicehaving the diffraction grating, a value of κL can be reduced withmanufacturing restrictions satisfied. Furthermore, with an effect thatthe reactive product deteriorated in crystallinity on a regrowth surfaceis removed, an improvement of the optical output efficiency of thesemiconductor laser device and a reduction of oscillation threshold canbe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing describing a diffraction grating having a sine waveshape in a semiconductor laser device according to a preferredembodiment of the present invention.

FIG. 2 is a drawing describing a relation between a thickness ofdiffraction grating layer and L₁/L₀ corresponding to an etching methodin the semiconductor laser device according to a preferred embodiment ofthe present invention.

FIG. 3A is a drawing that depicts a method of manufacturing asemiconductor laser device according to a first preferred embodiment ofthe present invention.

FIG. 3B is a drawing that depicts a method of manufacturing asemiconductor laser device according to a first preferred embodiment ofthe present invention.

FIG. 3C is a drawing that depicts a method of manufacturing asemiconductor laser device according to a first preferred embodiment ofthe present invention.

FIG. 3D is a drawing that depicts a method of manufacturing asemiconductor laser device according to a first preferred embodiment ofthe present invention.

FIG. 4A is a drawing that depicts a method of manufacturing asemiconductor laser device according to a second preferred embodiment ofthe present invention.

FIG. 4B is a drawing that depicts a method of manufacturing asemiconductor laser device according to a second preferred embodiment ofthe present invention.

FIG. 4C is a drawing that depicts a method of manufacturing asemiconductor laser device according to a second preferred embodiment ofthe present invention.

FIG. 5 is a drawing describing a relation between a height of adiffraction grating layer and κL corresponding to a shape of adiffraction grating of a semiconductor laser device in problems to besolved by the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments according to the present invention are described in detailbelow based on the drawings. Here, for description of the embodiments,in all drawings, the same components are provided with the same symbols,and are not repeatedly described herein.

Concept of the Embodiments

In the present embodiment, in order to reduce the value of κ, as a firstmeans, the diffraction grating after regrowth of a second clad layer ismade to have a sine wave shape. Ideally, the sine wave shape isrepresented by a perfect sine curve. In actuality, however, the sinewave shape is in a state that a rectangular shape remains to someextent.

Here, as parameters that represent a sine wave shape, as shown in FIG.1, a ratio (L₁/L₀) of an upper side (L₁) to a bottom side (L₀) parallelto a (100) surface in diffraction gratings (diffraction grating layers107), which formed in stripes perpendicular to a optical waveguidedirection have side walls each having at least two or more crystalfaces, is defined. If L₁/L₀ is 1, the shape of the diffraction gratingis a square. If L₁/L₀ is 0, the shape is a triangle. If L₁/L₀ is equalto or smaller than 0.3, the shape can nearly approximate to a sine wave.Therefore, it can be said that the sine wave shape is achieved whenL₁/L₀ is 0 to 0.3.

To realize this first means, in-situ vapor phase etching introducinghalogen-based gas to a reactor is performed after setting the height ofthe diffraction grating after dry and wet etching at 20 to 30 nm whichhas sufficient process controllability and increasing the temperature toa regrowth temperature of a clad layer.

As shown in FIG. 2, if the diffraction grating is formed through dry andwet etching, the height of the diffraction grating has to be 35 nm to 45nm to achieve L₁/L₀ equal to or smaller than 0.3 attaining a sine waveshape. With that height, the value of κL becomes larger. The reason forthis relation is that since only one crystal face appears throughanisotropic wet etching, the relation is uniquely determined by theheight of the diffraction grating layer and the bottom side length.

By contrast, when vapor phase etching is applied, two or more crystalfaces appear by etching the side walls of the diffraction grating layer.Therefore, even with the height of 15 nm to 30 nm, L₁/L₀ can be equal toor smaller than 0.3. With this scheme, the thickness of diffractiongrating after burying regrowth is reduced to 15 nm or lower, which hasbeen difficult with the conventional process, and a sine wave shape canbe easily achieved.

Furthermore, this scheme has an effect of cleaning, that is, thereactive product formed due to mass transport at an increase oftemperature is removed with halogen-based gas. Therefore, not onlyimproving optical output by reducing κ, but also oscillation thresholdcurrent and device reliability can be improved.

As a second means, to facilitate the above-described vapor phaseetching, the diffraction grating is formed of a III-V family compoundsemiconductor layer including at least one of In, Ga, As, and Pelements.

First Embodiment

FIG. 3 is a drawing that shows a process chart of a semiconductor laserdevice manufacturing method according to a first embodiment of thepresent invention.

On an n-InP substrate 101, an n-InP first clad layer 102, an n-InGaAlAsfirst optical guide layer 103, an InGaAlAs active layer 104, ap-InGaAlAs second optical guide layer 105, a p-InP spacer layer 106, ap-InGaAsP diffraction grating layer 107, and a p-InP cap layer 108 arelaminated through MOCVD (FIG. 3A). To enhance a carrier confinementeffect, the InGaAlAs active layer 104 has an MQW configuration consistedof an InGaAlAs barrier layer and InGaAlAs well layer. Also, inconsideration of controllability over a process of forming a diffractiongrating, the diffraction grating layer is made to have a film thicknessof 25 nm.

Next, through CVD, an insulating film 109, such as a silicon dioxide(SiO₂) film or a silicon nitride (SiN) film and the like, is formed.After a resist film 110 is applied, stripes with a period ofapproximately 200 nm in a direction perpendicular to a waveguide isformed through EB exposure or interference exposure. Using this resistfilm 110 as a mask, portions of the insulating film 109 at openings arethen removed through chemical etching. Chemical etching may be eitherone of wet etching using hydrofluoric acid mixed solution and dryetching using fluorinated gas (FIG. 3B).

Then, after removing the resist film 110 with solvent, portions of thep-InP cap layer 108 and the p-InGaAsP diffraction grating layer 107 atopenings are removed through dry etching using the insulating film 109as a mask. By using dry etching, better controllability in a depthdirection can be achieved compared with wet etching. Furthermore,etching is performed in a perpendicular shape from end of a mask of theinsulating film. Therefore, controllability over a duty ratio of thediffraction grating is improved, and controllability over κ is alsoimproved. Then, the insulating film 109 is removed with hydrofluoricacid mixed solution. With the process so far, a rectangular-shapeddiffraction grating including the p-InP cap layer on the active layerserved as a waveguide is formed (FIG. 3C).

Next, a surface treatment is performed on the diffraction gratingformation substrate with sulfuric acid mixed solution. Then, throughMOCVD, on the above-mentioned substrate, regrowth of a p-InP second cladlayer 111, a p-InGaAsP first contact layer 112, and a p-InGaAs secondcontact layer 113 is performed. Here, immediately before regrowth of thep-InP second clad layer 111, hydrochloric acid (HCl) gas is supplied tothe reactor to perform in-situ vapor phase etching on the surface of thep-InP cap layer 108 and the p-InGaAsP diffraction grating layer 107(FIG. 3D). At etching process, temperature of the substrate surface isset at 500° C., and the etching time is set so that the height of thediffraction grating is 13 nm.

In this vapor phase etching, it is possible to remove reactive productwith deteriorated crystallinity, formed on the side surfaces and trenchportions of the diffraction grating due to mass transport from the p-InPcap layer 108 in the process of increasing temperature. With this, aleak current caused by such a reactive product can be suppressed and anoscillation threshold of the semiconductor laser device can be reduced.Furthermore, by partially etching the p-InGaAsP diffraction gratinglayer 107, the height of the diffraction grating is reduced, and a sinewave shape can be achieved. Thus, value of κ is reduced, and opticaloutput efficiency is improved.

Here, in the present embodiment, HCl is used for etching of thediffraction grating formation substrate. Alternatively, halogen-basedgas containing a halogen element, such as methyl chloride (CH₃Cl),carbon tetrachloride (CCl₄), or carbon tetrabromide (CBr₄), can be used.

After burying regrowth to the diffraction grating formation substrate,processes using known technology described below is performed until chipmaking, thus, a semiconductor laser device is manufactured.

First, stripes of approximately 2 μm made of insulating film are formed.Using the insulating film as a mask, the regrowth layer is removedthrough wet etching and dry etching to form an optical waveguide. Afterremoving the stripe-shaped insulating film, an insulating film is formedagain on the entire surface. Then, only a current injecting portion ofthe optical waveguide is opened through photolithography and etching,and then EB vapor deposition and heat treatment are performed to form ap-side electrode. After polishing the back surface of the substrate to athickness of 100 μm, an n-side electrode is formed on the back surfacethrough vapor deposition. After that, a wafer is cut open in a bar shapeso that the oscillator length (L) is 500 μm. Then, through spattering,an end face is coated with a reflective film. Finally, the substrate ismade into a chip having device with width of 200 μm, thus, a DFBsemiconductor laser device is manufactured.

Through in-situ vapor phase etching using halogen-based gas immediatelybefore regrowth of the p-InP second clad layer 111 according to theembodiment, a reactive product with deteriorated crystallinity, formedin the trench portions of the diffraction grating due to mass transportat an increase of temperature for regrowth, is removed. Furthermore asine wave shape can be achieved with a reduced height of the diffractiongrading. With this, an effect of cleaning of the regrowth interface canbe achieved. Still further, by achieving reduction of κL n thesemiconductor laser device, an oscillation threshold of 3 mA is reduced.According to the present embodiment, a device characteristic yield, 70%in conventional, is improved to 90%.

Second Embodiment

FIG. 4 is a drawing that shows a process chart of a semiconductor laserdevice manufacturing method according to a second embodiment of thepresent invention. In this embodiment, a method of manufacturing asemiconductor laser device, in which a diffraction grating is disposedon a lower side of an active layer, is described.

On an n-InP substrate 201, an n-InP first clad layer 202, an n-InGaAlAsPdiffraction grating layer 203, and an n-InP cap layer 204 are laminatedthrough MOCVD (FIG. 4A). Here, the n-InGaAsP diffraction grating layer203 is made to have a film thickness of 25 nm, similar to that of thefirst embodiment.

Next, an insulating film is formed through CVD and, a diffractiongrating is formed using a process similar to that of the firstembodiment (FIG. 4B).

After that, through MOCVD, burying regrowth of the diffraction gratingis performed. Immediately before regrowth, as with the first embodiment,halogen-based gas is supplied so that the side surfaces and the trenchportions of the diffraction grating are etched. Then, an n-InP secondclad layer 205, an n-InGaAlAs first optical guide layer 206, an InGaAlAsactive layer 207, a p-InGaAlAs second optical guide layer 208, a p-InPthird clad layer 209, a p-InGaAsP first contact layer 210, and ap-InGaAs second contact layer 211 are successively deposited (FIG. 4C).

After that, from a mesa formation process to chip making, a proceduresimilar to that in the first embodiment is performed. Also in thepresent embodiment, the device characteristic yield, 60% inconventional, is improved to 90%.

Thus, while the invention carried out by the present inventors have beenspecifically described based on the embodiment, the present invention isnot limited to the above described embodiment, but it goes withoutsaying that various modifications are possible within the scope of theinvention.

The present invention relates to a semiconductor laser device andmanufacturing technology thereof, and particularly to a technologyeffective when applied to a distributed-feedback ordistributed-reflective semiconductor laser device for use in apparatusesfor optical transmission or for information storage, and a method ofmanufacturing such a semiconductor laser device.

1. A distributed-feedback or distributed-reflective semiconductor laserdevice comprising: a diffraction grating formed in stripes perpendicularto a waveguide direction, wherein the diffraction grating has side wallseach having at least two or more crystal faces, and a ratio of length ofan upper side in a waveguide direction to a bottom side parallel to a(100) surface is 0 to 0.3.
 2. The semiconductor laser device accordingto claim 1, wherein the diffraction grating is composed of a III-Vfamily compound semiconductor layer including at least one of In, Ga,As, and P elements.
 3. A method of manufacturing semiconductor laserdevice comprising: a first step of laminating a first clad layer, anactive layer, and a diffraction grating layer on a semiconductorsubstrate through epitaxial growth; a second step of forming adiffraction grating by etching the diffraction grating layer; and athird step of performing burying regrowth of the diffraction gratingthrough epitaxial growth on a second clad layer of different conductiontype from the first clad layer, wherein immediately before the thirdstep, halogen-based gas is introduced to a reactor, and etching isperformed on the diffraction grating so that each side wall has at leasttwo or more crystal faces and a ratio of length of an upper side in awaveguide direction to a bottom side parallel to a (100) surface is 0 to0.3, and a reactive product formed on side surfaces of the diffractiongrating and in trench portions between stripes of the diffractiongrating at an increase in temperature for regrowth is removed.
 4. Themethod of manufacturing semiconductor laser device according to claim 3,wherein in the first step, a first optical guide layer is further formedbetween the first clad layer and the active layer, and a second opticalguide layer and a spacer layer are formed between the active layer andthe diffraction grating layer.
 5. The method of manufacturingsemiconductor laser device according to claim 3, wherein the diffractiongrating layer is a III-V family compound semiconductor layer includingat least one of In, Ga, As, and P elements.
 6. A method of manufacturingsemiconductor laser device comprising: a first step of laminating,through epitaxial growth, a first clad layer and a diffraction gratinglayer on a semiconductor substrate; a second step of forming adiffraction grating through etching on the diffraction grating layer;and a third step of burying the diffraction grating through epitaxialgrowth on a second clad layer of identical conduction type to the firstclad layer for regrowth of the active layer, wherein immediately beforethe third step, halogen-based gas is introduced to a reactor, etching isperformed on the diffraction grating so that each side wall has at leasttwo or more crystal faces and a ratio of length of an upper side in awaveguide direction to a bottom side parallel to a (100) surface is 0 to0.3, and a reactive product formed on side surfaces of the diffractiongrating and in trench portions between stripes of the diffractiongrating at an increase in temperature for regrowth is removed.
 7. Themethod of manufacturing semiconductor laser device according to claim 6,wherein in the third step, a first optical guide layer is further formedbetween the second clad layer and the active layer, and a second opticalguide layer and a third clad layer are formed on an upper portion of theactive layer.
 8. The method of manufacturing semiconductor laser deviceaccording to claim 6, wherein the diffraction grating layer is a III-Vfamily compound semiconductor layer including at least one of In, Ga,As, and P elements.